humm well the way I was taught is that all other souls will be saved because of the great experiement (here) between good and evil. God was calling Satan's bluff. So we live with sin.
Well, I think there may be validity to that (even though it's really just religious speculation-it's not in the Bible). But I think that it's more likely the souls that would benefit from this "experiment" (which I think of as more of a "demonstration" since God already knows the outcome), would be beings from another universe or dimension--not one we could see on get to by travelling through space. If God can create billions of suns and planets he can also create billions of universes. A straightforward reading of the Bible shows that the earth was created before the moon and stars, which were created "to give light" and "to mark times and seasons and days and years."
BTW the lyrics I quoted re from the song "U.F.O." by the Christian singer Larry Norman.